Question: Emily is 3 times as old as Christopher and is also 10 years older than Christopher. How old is Emily?
Explanation: We can use the given information to write down two equations that describe the ages of Emily and Christopher. Let Emily's current age be $e$ and Christopher's current age be $c$ $e = 3c$ $e = c + 10$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $e$ is to solve the second equation for $c$ and substitute that value into the first equation. Solving our second equation for $c$ , we get: $c = e - 10$ . Substituting this into our first equation, we get the equation: $e = 3$ $(e - 10)$ which combines the information about $e$ from both of our original equations. Simplifying the right side of this equation, we get: $e = 3e - 30$ Solving for $e$ , we get: $2 e = 30$ $e = 15$.